Doping and Electrochemical Capacitance of Carbon Nanotube-Polypyrrole Composite Films
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Doping and Electrochemical Capacitance of Carbon Nanotube-Polypyrrole Composite Films Mark Hughes, George Z. Chen, Milo S. P. Shaffer, Derek J. Fray and Alan H. Windle Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK. ABSTRACT Composite films were electrochemically synthesised via the simultaneous deposition of multiwalled nanotubes and polypyrrole, a conducting polymer. Negatively charged functional groups attached to the surface of the acid treated nanotubes enables the tubes to act as a dopant for the PPy in these films. Scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy revealed that the nano-porous three-dimensional arrangement of PPy coated MWNTs in these films produced specific capacitances per mass and geometric area as high as 192 F g-1 and 1.0 F cm-2, respectively. This value of specific capacitance per geometric area exceeds that of both component materials and other carbon nanotube-conducting polymer composites. The composite films described in this report were also able to charge and discharge more than an order of magnitude faster than similarly prepared pure PPy films. The nano-porosity and small diffusion distances within the composite films, crucial to achieving the superior capacitive performance, were found to be dependent on the concentration of nanotubes and additional dopant anions in the polymerisation electrolyte, offering possibilities for tailoring of the composite structure. INTRODUCTION Growing demands in industries such as transport and communication for electrical energy storage devices that can deliver high power level pulses have prompted considerable interest in electrochemical capacitors, or supercapacitors. While carbon nanotubes and conducting polymers have separately received considerable attention as supercapacitive materials, only recently has the capacitive performance of composites made by combining these two materials been reported, revealing capacitances greater than that of either component material.[1-4] Composites of carbon nanotubes and conducting polymers such as polypyrrole, polyaniline and poly(p-phenylene vinylene) can be grown by simply mixing the polymer and nanotubes,[5-7] or using a variety of chemical[8,9] and electrochemical[4,10-12] polymerisation techniques. The superior supercapacitive performance of carbon nanotube-conducting polymer composites is attributed to their nanoporous structure which combines the three-dimension charge storage capabilities of redox active conducting polymers, with the high surface area and electrical conductivity of carbon nanotubes. Here we report the supercapacitive properties of a particular carbon nanotube-polypyrrole composite in which the conducting polymer is doped by the embedded nanotubes. EXPERIMENTAL DETAILS Multi-walled carbon nanotubes (MWNTs) and polypyrrole (PPy) were simultaneously deposited onto a graphite disk working electrode (0.33 cm2) using an oxidising potential of 0.7 V measured against a saturated calom
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